FIG. 16 shows a structure of a conventional scribing head 50, and FIG. 17 is a side view thereof. A tip holder 52 permits rotation of a cutter wheel tip 51. This tip holder 52 can swing freely via a vertical shaft bearing 55 which is accommodated in a holder carrier 54. Accordingly, when the scribing head 50 moves (to the right in FIG. 16), the tip holder 52 swings to align with the direction of this movement.
A scribing unit 56 is disposed above the holder carrier 54, with a small gap G. On the right side of the holder carrier 54, a bearing 57 is embedded at a predetermined position, orthogonally to the drawing sheet. A center shaft 57a of the bearing 57 is integrated with the scribing unit 56. On the left side, the bottom end of the holder carrier 54 is checked by a stopper 53. Accordingly, the holder carrier 54 pivots about the bearing 57, within a range permitted by the gap G.
The scribing unit 56 contains an air cylinder chamber 58 which extends vertically and in which a piston 59 is inserted. The bottom end of the piston 59 has a recess for keeping a bearing 60 in a loose-fit manner. Its center shaft 60a is held by the piston 59. Accordingly, the peripheral body of the bearing 60 rotates freely, with its bottom end touching the top part of the holder carrier 54. When air is fed to this air cylinder chamber 58 at a prearranged pressure, the piston 59 and the bearing 60 are pressed down to apply a predetermined scribe pressure (scribe load) to the cutter wheel tip 51. Even when the holder carrier 54 is tilted, the bearing 60 transmits the pressure from the piston 59 straight down to the holder carrier 54 without fail.
Turning to FIG. 17, the scribing head 50 is disposed movably along a horizontal guide rail 67 of the scribing apparatus 66. The scribing head 50 can also move upwardly and downwardly when driven by an up-down cylinder or motor 65. Descent of the scribing head 50 causes the cutter wheel tip 51 to abut on a glass plate W. After that, the holder carrier 54 pivots about the bearing 57, creating a clearance between the holder carrier 54 and the stopper 53. On detection of this clearance, descent of the scribing head 50 stops. Then, the scribing head 50 descends again by a predetermined cut-in depth. Thereafter, a predetermined scribe pressure is set to the air cylinder chamber 58.
FIG. 18 shows a plate glass scribing apparatus 20 which is disclosed in Japanese Patent Laid-open Publication No. H8-225333. This apparatus has a detection unit 24 which employs a piezoelectric device for detecting up-down movement of a glass cutter 38 and a desired scribe pressure. A detection signal from the piezoelectric device is processed through an amplification unit 39 and a process unit 26, and a control unit 28 controls a linear motor 22.
Having said that, the scribing head 50 of FIG. 16 requires complex mechanisms such as a motor for lowering the cutter wheel tip 51 to a certain level, and an electric-pneumatic converter for setting a desired scribe pressure. Likewise, the apparatus of FIG. 18 needs the detection unit 24 and a circuit for processing a detection signal from this unit, thus complicating the scribing head mechanism. Further, because the movable members have large inertia, the apparatus shows poor responsivity and has difficulty in stabilizing the scribing quality.
Incidentally, to produce square glass chips which are used as an electric parts material, a base material of a large glass plate is scribed and broken into square glass pieces in the following manner. In the beginning, a cutter wheel tip is made to run in one direction on a surface of the base material. To form parallel scribe lines, this operation is repeated for predetermined times, with the start position displaced after each run. Next, to form scribe lines which intersect mutually, the running direction of the cutter wheel tip is changed so as to cross the previous running direction. After this cross-scribing operation, the base material is transferred to a breaking machine. The breaking machine imposes a certain pressure on the base material and applies a bending moment along the scribe lines formed on the base material. Eventually, the base material is broken along the scribe lines to give intended square glass chips.
As known, this scribing operation can be performed, for example, with an apparatus illustrated in FIG. 19. In the following description, the lateral directions in this drawing are taken as X directions and the directions orthogonal to the drawing sheet are taken as Y directions.
This scribing apparatus comprises: a worktable 70 which can rotate horizontally and on which a glass plate GL is laid and fixed by a vacuum suction means; a pair of guide rails 71, 71 which support the worktable 70 movably in Y directions; a ball screw 72 which displaces the worktable 70 along the guide rails 71, 71; a guide bar 73 which is constructed above the worktable 70 along X directions; a scribing head 76 which is attached to the guide bar 73 and slidable in X directions; a motor 74 for sliding the scribing head 76; a tip holder 77 which is swingably attached at the bottom of the scribing head 76 and which is movable upwardly and downwardly; a cutter wheel tip 78 which is rotatably mounted at the bottom end of the tip holder 77; and a pair of CCD cameras 75 which locate above the guide bar 73 in order to recognize alignment marks on the glass plate GL laid on the worktable 70.
While the scribing head is running, various factors, such as inevitable microscopic unevenness on the surface of the glass plate GL, may cause distortion of a scribe line. Hence, an anti-distortion measure is incorporated into the scribing head of the scribing apparatus of this structure. As specifically shown in FIG. 20, the tip holder 77 is mounted to the scribing head body 76A via a turning shaft 79 which extends orthogonally to the surface of the glass plate GL, such that the tip holder 77 can swing freely around the axis of the turning shaft 79. In addition, the cutter wheel tip 78 is attached to the tip holder 77 at the position Q2 which is offset from the axis Q1 of the turning shaft 79 toward the opposite direction to the running direction (direction of Arrow S in the drawing). As a result, while the scribing head is running, the cutter wheel tip 78 follows the scribing head body 76A. Hence, the cutter wheel tip 78 gains stability in straight movement, which serves to prevent distortion of a scribe line.
This scribing apparatus operates without problem as far as scribe lines are formed on a glass plate only in one direction. On the other hand, referring to FIG. 21, a cross-scribing operation tends to fail near the points where the cutter wheel tip 78 crosses and passes the former scribe lines L1-L3, because the apparatus does not make the latter scribe lines L4-L6 at those points, or it “skips” the intersections. If intersections are skipped on a scribed glass plate, the glass plate is not broken precisely along the scribe lines in the breaking operation using the above-mentioned breaking machine. Eventually, the scribing apparatus yields a volume of defective products and shows an extremely poor productivity.
The reason for this problem is understood as follows: a scribing force which is applied from the scribing head to the glass plate surface is cancelled when the cutter wheel tip crosses and passes existing scribe lines, by latent internal stresses on both sides of these scribe lines.
As a solution to this problem, the applicant proposed a scribing method, a scribing head and a scribing apparatus (Japanese Patent Application No. 2000-142969). The scribing head comprises: a scribing head body which runs on a brittle substrate; a tip holder mounted on the scribing head body via a turning shaft which extends orthogonally to a surface of the brittle substrate, the tip holder being freely swingable around the axis of the turning shaft; a cutter wheel tip attached to the tip holder at a position which is offset from the axis of the turning shaft toward the opposite direction to the running direction. This scribing head is used to provide scribe lines which cross each other on the surface of the brittle substrate. During the scribing operation, the swing range of the tip holder is controlled at greater than 0° but not greater than 2°. FIG. 22 shows a front view of an embodiment of this scribing head, and FIG. 23 is its bottom view.
The scribing head has a scribing head body 80, a bearing housing 81, a tip holder 82, a cutter wheel tip 83, and a bias means 84.
The bottom of the scribing head body 80 is cut away to form a notch 85 which accommodates the bearing housing 81. An end of the bearing housing 81 is joined, via a bearing 87, with a horizontal support shaft 86 which is inserted in the scribing head body 80. The other end abuts on a stopper shaft 88 which is contained within the scribing head body 80 and which extends parallel to the support shaft 86. Hence, the bearing housing 81 pivots around the axis of the support shaft 86 until it is stopped by the stopper shaft 88.
The tip holder 82 is mounted to the bearing housing 81 via a turning shaft 89 which extends orthogonally to the surface of the brittle substrate, the tip holder being freely swingable around the axis of the turning shaft 89. A bearing 40 is set between the turning shaft 89 and the bearing housing 81. The bias means 84 which locates above the turning shaft 89 is arranged to apply a biasing force to the cutter wheel tip 83, through the turning shaft 89 and the tip holder 82.
The cutter wheel tip 83, attached to the tip holder 82, is offset from the axis of the turning shaft 89 toward the opposite direction to the running direction S of the scribing head (offset to the left in FIG. 22).
During the scribing operation, the swing range A of the tip holder 82 is controlled at greater than 0° but not greater than 2°, by means of a groove 41 which is formed in the bottom surface of the bearing housing 81. Namely, the tip holder 82 has its upper end accommodated in the groove 41 of the bearing housing 81. When the tip holder 82 swings to the maximum limit of its swing range, either pair of opposing corners 42, 45 (43, 44) at its upper end are arranged to abut on interior walls 46, 47 of the groove 41. Owing to this arrangement, the swing range A of the tip holder 82 can be adjusted in the defined range, by adjustment of clearances between the interior surfaces 46, 47 of the groove 41 and the side faces 48, 49 at the upper end of the tip holder 82. It is readily understood that the clearances are set greater in order to expand the swing range A, whereas the clearances are set smaller for a narrower swing range.
These arrangements ensure the operation of the scribing head proposed by the applicant, by securing the swing action of the tip holder to such a degree as to keep straight movement of the cutter wheel tip, and also by suppressing the influence of latent internal stresses near the intersections. Consequently, even if a pressure is applied by the scribing head at a fixed level, the cross-scribing operation does not experience skipping of intersections nor missing of a scribe line at the starting end of scribing. Thus, the applicant's scribing head achieves the desired objects.
In this scribing head, the cutter wheel tip, attached to the tip holder, is offset from the axis of the turning shaft toward the opposite direction to the running direction. During the scribing operation, the scribing head runs with the support shaft side ahead. Hence, the cutter wheel tip is caused to jump up when the scribing head crosses existing scribe lines, or passes an undulated or warped part of a glass or an uneven part on a glass surface. In this connection, the tip holder tends to pivot about the support shaft and to bounce over the glass surface. The schematic view of FIG. 13 illustratively explains this phenomenon, wherein the sign GL designates glass, 83 indicates the cutter wheel tip, and 86 indicates the support shaft.
Namely, when the scribing head runs (in the direction of Arrow S in the drawing) with the support shaft 86 ahead and with the cutter wheel tip 83 being pressed against a surfact of the glass GL by the bias means 84, the point of contact between a blade ridge 83A of the cutter wheel tip 83 and the surface of the glass GL is given as the point P. At this point P, a reaction force R is generated toward the center of the cutter wheel tip 83, against a resultant force of a horizontal scribing force component M and a vertical scribing force component N, wherein the scribing force components M and N represent a horizontal component and a vertical component, respectively, of a scribing force which is required to scribe the glass GL by the cutter wheel tip 83. The reaction force R acts on the cutter wheel tip 83, as a turning moment around the support shaft 86. Consequently, the cutter wheel tip 83 is caused to jump up. In this connection, the tip holder (not shown) tends to pivot about the support shaft 86 and to bounce over the glass surface GL.
If the tip holder bounces in this manner, the pressure to the cutter wheel tip 83 is cancelled by the reaction force R. In this situation, formation of a deep vertical crack is less likely.
Incidentally, let us describe a mechanism of how the cutter wheel tip creates a vertical crack on the glass. For a start, a load imposed on the blade edge causes elastic deformation on the glass surface, at a part where the blade edge touches the glass surface. With an increase of the load on the blade edge, this part undergoes plastic deformation. When the blade edge load becomes so great as to exceed the critical limit of plastic deformation, brittle fracture occurs, and a vertical crack begins to grow in the glass thickness direction. Growth of the vertical crack terminates once the leading end of the crack reaches a certain depth (a distance from the surface of the brittle substrate) which depends on the amount of blade edge load, glass composition, glass thickness, etc. In this case, provided that the composition and the thickness of the glass are the same, the depth of the leading end of the vertical crack (hereinafter mentioned as “vertical crack propagation depth”) is controllable only by the load on the blade edge. In other words, with an increase of the blade edge load, the blade edge of the cutter wheel tip cuts deeper into the glass surface and gives a greater energy to generate a vertical crack, so that the vertical crack propagation depth becomes longer. However, once the blade edge load exceeds a certain level, a comparatively deep vertical crack is obtained, but at the same time, internal distortion which has accumulated near the glass surface reaches saturation. Such an excessive blade edge load results in growth of a so-called horizontal crack in a direction totally different from the growing direction of the vertical crack. The horizontal crack causes generation of a large amount of undesirable chips.
The inventors investigated the above-mentioned mechanism in more detail and discovered a relationship between the blade edge load and the vertical crack propagation depth, as given in FIG. 14. As seen in the graph of FIG. 14, the vertical crack propagation depth is related with three stages: an initial stage (Stage A) where the depth gently increases with increment of the blade edge load; an intermediate stage (Stage B) where the depth sharply increases with increment of the blade edge load; and a final stage (Stage C) where the depth hardly increases despite increment of the blade edge load. While a horizontal crack is not observed in Stage A and Stage B, Stage C showed drastic increase of horizontal cracks.
Based on this knowledge, the inventors discovered that a deep vertical crack is obtainable without generation of a horizontal crack, when a scribing operation is performed with a blade edge load in Stage B, where the propagation depth increases sharply with increment of the blade edge load.
Nevertheless, due to the extreme narrowness of the range of the blade edge load in Stage B, it turned out to be difficult to achieve a stable scribing operation in Stage B alone as far as adjustment of the blade edge load is done as in a usual scribing operation. In particular, as discussed above, prior art cannot prevent a bounce of the tip holder, permitting the pressure to the cutter wheel tip to be cancelled by the reaction force R. Under such circumstances, it is awfully difficult to adjust the blade edge load within the extremely narrow Stage B.
Also as mentioned earlier, the cross-scribing operation involves a task of preventing skipping of intersections. For this purpose, the blade edge load for formation of second scribe lines should be much greater than the load for formation of first scribe lines. In this case, the blade edge load often falls into Stage C, inevitably causing increase of horizontal cracks and associated generation of a large volume of chips.
In addition to the problems mentioned above, a scribing operation using a conventional cutter wheel is also affected by some external factors such as an undulated or warped glass, an uneven glass surface, and wearing of the tip holder which holds the cutter wheel tip or of the scribing head which carries the tip holder. In this case, formation of stable scribe lines is often hampered.
This invention is made to solve these problems. A first object of the invention is to provide a scribing head, and a scribing apparatus and a scribing method using this scribing head, in which the scribing head has a simple mechanism and is suitably adaptable to various scribe conditions. A second object of the invention is to provide a scribing head, and a scribing apparatus and a scribing method using this scribing head, in which the scribing head prevents not only skipping of intersections during a cross-scribing operation but also a bounce of the tip holder. As a consequence, a pressure imposed on the cutter wheel tip is efficiently applied to a brittle substrate, realizing a vertical crack which is much deeper than the one obtained in a conventional manner.